#define _CRT_SECURE_NO_WARNINGS 1
class Solution {
public:
    int longestArithSeqLength(vector<int>& nums)
    {
        int n = nums.size();
        vector<vector<int>> dp(n, vector<int>(n, 2));

        unordered_map<int, int> hash;
        hash[nums[0]] = 0;
        int ret = 2;
        for (int i = 1; i < n; i++)
        {
            for (int j = i + 1; j < n; j++)
            {
                int x = 2 * nums[i] - nums[j];
                if (hash.count(x))
                    dp[i][j] = dp[hash[x]][i] + 1;
                ret = max(ret, dp[i][j]);
            }
            hash[nums[i]] = i;
        }

        return ret;
    }
};